1. Field of the Invention
The present invention relates to methods and apparatus for completing a well. Particularly, the present invention relates to an apparatus for cleaning an inner surface of a casing. More particularly, the present invention relates to a packer having a cleaning device for cleaning the inner surface of the casing.
2. Description of the Related Art
In the drilling of oil and gas wells, a wellbore is formed using a drill bit that is urged downwardly at a lower end of a drill string. After drilling a predetermined depth, the drill string and bit are removed and the wellbore is lined with a string of casing. An annular area is thus formed between the string of casing and the formation. A cementing operation is then conducted in order to fill the annular area with cement. The combination of cement and casing strengthens the wellbore and facilitates the isolation of certain areas of the formation behind the casing for the production of hydrocarbons.
It is common to employ more than one string of casing in a wellbore. In this respect, a first string of casing is set in the wellbore when the well is drilled to a first designated depth. The first string of casing is hung from the surface, and then cement is circulated into the annulus behind the casing. The well is then drilled to a second designated depth, and a second string of casing, or liner, is run into the well. The second string is set at a depth such that the upper portion of the second string of casing overlaps with the lower portion of the upper string of casing. The second “liner” string is then fixed or “hung” off of the upper surface casing. Afterwards, the liner is also cemented. This process is typically repeated with additional liner strings until the well has been drilled to total depth. In this manner, wells are typically formed with two or more strings of casing of an ever-decreasing diameter.
The process of hanging a liner off of a string of surface casing or other upper casing string involves the use of a liner hanger. In practice, the liner hanger is run into the wellbore above the liner string itself. The liner hanger is actuated once the liner is positioned at the appropriate depth within the wellbore. The liner hanger is typically set through actuation of slips which ride outwardly on cones in order to frictionally engage the surrounding string of casing. The liner hanger operates to suspend the liner from the casing string. However, it does not provide a fluid seal between the liner and the casing. Accordingly, it is desirable in many wellbore completions to also provide a packer.
During the wellbore completion process, the packer is run into the wellbore above the liner hanger. A threaded connection typically connects the bottom of the packer to the top of the liner hanger. Known packers employ a mechanical or hydraulic force in order to expand a packing element outwardly from the body of the packer into the annular region defined between the packer and the surrounding casing string. In addition, a cone is driven behind a tapered slip to force the slip into the surrounding casing wall and to prevent packer movement. Numerous arrangements have been derived in order to accomplish these results.
A problem associated with most conventional packer systems is the potential for the packer to leak. Generally, packers are designed to be installed on clean or smooth surfaces. Thus, it is desirable for the surrounding casing to have a clean contact surface for engagement with a packer. A packer installed on a casing having a dirty or rough surface may not initiate a seal or may become unseated over time because the dirty surface contains contaminants that affect the seal between the packer and the casing.
The contact surface of a surrounding casing in a wellbore typically contains deposits or contaminants left behind from the multitude of wellbore operations that are performed prior to the setting of the packer. For instance, cementing operations are commonly performed to fill the annular void between the casing and the formation. The cement is initially pumped down the inner diameter of the casing and pushed out and around the bottom of the casing. The cement slurry is typically followed by a wiper plug to wipe the cement from the inner surface of the casing. However, the wiping process is not totally efficient at high pump rates, and the fins of the wiper plug typically wear down during the process. As a result, a thin sheath of cement may be left on the inner surface of the casing. Additionally, the drilling mud used to displace the cement may also leave a layer of dehydrated solids on the inner surface.
Other sources of contaminants include the cuttings from the formation, debris from drilling out the wiper plug and the remaining cement in the inner diameter. These cuttings may remain in the casing if the circulating process is not efficient. In deviated or horizontal wells, any remaining cuttings may settle on the low side of the wellbore when the circulation is stopped. The cuttings may become embedded in the dehydrated solids on the casing wall.
Contaminants may also develop during the production of the casing. Casing is typically constructed from steel and steel alloys. The formation of the casing itself typically leaves behind a layer of oxidation products on the surface. The surface may form additional corrosion when it is exposed to moisture and oxygen. The corrosion process is accelerated when salt is present in the atmosphere, a condition often found in offshore drilling.
In summary, casing in the wellbore may have a multitude of contaminants on its inner surface. The contaminants may include oxidized products, corrosion, cuttings, cement, and dehydrated solids. These contaminants are generally somewhat porous, irregular in shape, or extrudable over time. As a result, the contaminants may compromise the sealing integrity of the packer.
Contaminants may be removed from the casing by performing a separate casing scraper run. However, scraper runs are generally not performed when running a liner in order to save time. Additionally, the time required to run a separate scraper run may cause the wellbore to fall in. Other methods of removing the contaminants include pumping spacer fluids or wash fluids in front of the cement to effect some cleaning action. However, fluid based cleaning methods are generally not as effective as mechanically based cleaning methods.
There is a need, therefore, for an apparatus for cleaning an inner surface of the casing. There is a further need for a packer having a cleaning device that cleans the inner surface of the casing without significantly adding to the time necessary for the wellbore operation. There is a further need for a method of cleaning the inner surface of the casing and setting the packer in one run.